Recently, a fixing apparatus for use in an electric image forming apparatus has been developed which is illustrated in FIG. 10 of the accompanying drawings as a technology disclosed in Japanese Patent Application Laid-Open Publication HEI 6-318001, for example, not employing a conventional heating roller type fixing system. As shown in FIG. 10, the fixing apparatus is provided with a fixing roller R1, a heating roller R3, and an endless fixing belt B trained around the rollers R1 and R3, and a pressing roller R2 disposed below and pressed against the fixing roller R1 with the fixing belt B interposed therebetween. In the fixing apparatus disclosed in the Laid-Open Publication, it is combined with a preheat of a recording medium D.
Since the recording medium D is preheated, the nipping region may be set to a relatively low temperature. The fixing belt B is of such a small heat capacity that when the recording medium D passes through the nipping region, the temperature of the fixing belt B is quickly lowered to increase the coherent ability of the toner which is separated from the fixing belt B at the outlet of the nipping region, for thereby allowing the toner to be easily separated from the fixing belt B. Even if the fixing belt B is free of oil or coated with a small amount of oil, a clear fixed toner image can be produced on the recording medium D without offsets. The fixing apparatus shown in FIG. 10 is thus capable of solving the problems of toner separation and oil coating, which have not been eliminated by other fixing apparatus using only a heating roller.
Here, the conventional fixing apparatus shown in FIG. 10 will be described in more detail. The fixing apparatus is provided with a fixing roller R1, a pressing roller R2, a heating and tensioning roller R3, and an endless fixing belt B trained around the rollers R1 and R3. There is provided an oil coating roller R4 disposed above the upper run of the fixing belt B, and a guide plate G for supporting the recording medium D disposed below a lower run of the fixing belt B with a predetermined gap therebetween. The gap between the guide plate B and the lower run of the fixing belt B serves as a preheating passage P for preheating the recording medium D.
There is provided a heater H which is housed in the heating and tensioning roller R3. Further, additional heater may be housed in fixing roller R1 and/or pressing roller R2. There is also provided a thermistor T which contacts a non-passage portion of the fixing belt B in order to control the temperature of the fixing belt B.
However, in the conventional belt-type fixing apparatus having above mentioned construction, it is preferable to rotate a driving roller for endlessly traveling the fixing belt B, which is rotatable about a stationary rotating axis. From the present point, it is constructed that the fixing roller R1 to which the pressing roller R2 is pressed is set to be rotatably driven. When it is limited that the fixing roller R1 is only set as the driving roller for endlessly traveling the fixing belt B, this causes certain restrictions of design for, for example, a power transmission path or route of the driving force, a location of a driving source, being thereby failing to ensure the freedom of the design. Therefore, it has been desired to obviate this drawback.
Accordingly, it is a primary object of the present invention to provide a fixing apparatus which is capable of ensuring the freedom of the design for a driving mechanism which is provided for endlessly traveling the fixing belt.